The present invention relates to a process for the production, by a moulding process, in particular a reaction injection moulding process (RIM process), of a micro-cellular or non-cellular elastomeric polyurethane skin layer, which usually has to be provided with a backing layer to form a composite or, more generally, to a process for the production of a composite comprising a first layer shaped part, which is made by a moulding process from a polyurethane reaction mixture, and a second layer shaped part which is adhered to the first part to form the composite. The first layer shaped part, in particular the skin layer, is made by injecting or pouring a polyurethane reaction mixture in a closed mould having first and second co-operating mould sections so that the back side of this first part is formed on the side of the second mould section.
In prior art methods for producing such moulded first parts, in particular moulded skins, use has to be made of so-called internal or external release agents in order to avoid sticking of the produced part, in particular the back thereof, to the mould surface or in other words to enable the produced part to be removed from the mould. Internal release agents are products incorporated in the reaction mixture to reduce the adhesion of the produced part to the mould while external release agents are products like waxes applied to the mould surface also to reduce the adhesion. A drawback of these known methods is that the release agents do not only reduce the adhesion of the produced part to the mould surface but they also reduce the adhesion of backing layers which are subsequently applied to the back of the produced layer shaped parts. In case of an elastomeric skin layer, these backing layers may for example be a foam layer in order to provide a soft touch. However, the moulded part itself may also be a foam layer, either with or without an integral skin, and the backing layer a rigid support layer.
An existing technique wherein the disadvantage of the reduced adhesion is avoided is disclosed in WO 93/23237 in the name of the present applicant. According to this known technique, a self-supporting synthetic trim part comprising an elastomeric polyurethane skin, a rigid synthetic carrier and a polyurethane foam layer between the skin and the carrier is produced by spraying, in a first step, a reaction mixture for the elastomeric skin against a mould surface and, in a second step, a reaction mixture for the foam layer against the back of the elastomeric skin. The carrier is then formed in situ against the back of the foam layer by a spray or a RIM process. As a result of the spray steps, no external release agents have to be applied to the back of the elastomeric skin or to the back of the foam layer so that a good adhesion can be obtained between the skin layer and the foam layer and between the foam layer and the synthetic carrier.
An object of the present invention is now to provide an alternative process wherein the first polyurethane part can be produced by a RIM or pouring process instead of by a spray process without necessarily reducing the adhesion of a subsequently applied backing layer as a consequence of the use of release agents.
To this end, the process according to the invention is characterised in that the surface of the second mould section is covered with a covering which is either removable from the second mould section and made of a flexible material or of a rigid, pre-shaped non-stick material, or which is a permanent non-stick coating layer and said reaction mixture is applied in the mould cavity between this covering and the surface of the first mould section.
In case the covering is removable from the second mould section and adheres to the back of the first part, it can be left on the back of this first part, in particular when the covering consists of a thin, flexible sheet material such as a polymeric film, whilst in case it does not or nearly not adhere to the first part, it can be removed therefrom. In both cases, no internal or external release agent has to be used to avoid adherence of the produced first part to the surface of the second mould section. When the covering is removed from the back of the moulded part, an external release agent may however be applied to the second mould section since in this case, the release agent will be removed from the moulded part when removing the covering. Instead of covering the surface of the second mould section with a flexible or rigid removable covering, it can also be covered with a permanent non-stick coating layer which adheres to the second mould surface. In this way, the same result is obtained, i.e. the second mould surface has also not to be coated with an external mould release agent to prevent the moulded part from adhering or sticking thereto. It should be noted that the term xe2x80x9cpermanentxe2x80x9d is not to be understood as meaning absolutely permanent but it only indicates that the non-stick layer is a rigid layer which remains on the mould surface when demoulding the moulded part so that it can be used for several hundreds or thousands or even more mouldings, depending on the wear properties of the coating.
An additional advantage of the process according to the invention is obtained when the second mould section is composed of at least two slides defining the surface of this second mould section. In this case, the provision of the removable covering or the permanent non-stick coating on the mould surface prevents the reaction mixture from penetrating partially between the slides, especially when the permanent non-stick coating is also applied on the lateral, mutually engaging sides of the slides. In this way, the slides have no longer to be cleaned, or at least less regularly, and further no burrs or flashes have to be removed from the back of the moulded part. Especially in the case of an elastomeric skin, the presence of burrs on the back side can not be tolerated since, at the location of these burrs, the backing layer does not adhere to the skin so that the skin may form unacceptable blisters.
In a particular embodiment of the process according to the invention, said covering is a flexible sheet material, in particular a film or a membrane, which adapts itself to the shape of the mould section.
The invention also relates to a mould for use in the process according to the invention as described hereabove and comprising at least first and second co-operating mould sections having opposing surfaces which co-operate to provide a mould cavity arranged to mould said first part therein, the second mould section being situated on the back side of first part. The mould is characterised in that the surface of the second mould section is covered with a covering which is either removable from this second mould section and made of a flexible material or of a rigid non-stick material, or which is a permanent non-stick coating layer and said the mould cavity is formed between this covering and the surface of the first mould section.